gh-143228: Fix UAF in perf trampoline during finalization

Include/internal/pycore_ceval.h

@@ -103,7 +103,6 @@ extern int _PyPerfTrampoline_SetCallbacks(_PyPerf_Callbacks *);
 extern void _PyPerfTrampoline_GetCallbacks(_PyPerf_Callbacks *);
 extern int _PyPerfTrampoline_Init(int activate);
 extern int _PyPerfTrampoline_Fini(void);
-extern void _PyPerfTrampoline_FreeArenas(void);
 extern int _PyIsPerfTrampolineActive(void);
 extern PyStatus _PyPerfTrampoline_AfterFork_Child(void);
 #ifdef PY_HAVE_PERF_TRAMPOLINE

Include/internal/pycore_interp_structs.h

@@ -87,7 +87,9 @@ struct _ceval_runtime_state {
         struct trampoline_api_st trampoline_api;
         FILE *map_file;
         Py_ssize_t persist_after_fork;
-       _PyFrameEvalFunction prev_eval_frame;
+        _PyFrameEvalFunction prev_eval_frame;
+        Py_ssize_t trampoline_refcount;
+        int code_watcher_id;
 #else
         int _not_used;
 #endif

Misc/NEWS.d/next/Core_and_Builtins/2025-12-27-23-57-43.gh-issue-143228.m3EF9E.rst

@@ -0,0 +1,4 @@
+Fix use-after-free in perf trampoline when toggling profiling while
+threads are running or during interpreter finalization with daemon threads
+active. The fix uses reference counting to ensure trampolines are not freed
+while any code object could still reference them. Pach by Pablo Galindo

Python/perf_trampoline.c

@@ -204,6 +204,42 @@ enum perf_trampoline_type {
 #define persist_after_fork _PyRuntime.ceval.perf.persist_after_fork
 #define perf_trampoline_type _PyRuntime.ceval.perf.perf_trampoline_type
 #define prev_eval_frame _PyRuntime.ceval.perf.prev_eval_frame
+#define trampoline_refcount _PyRuntime.ceval.perf.trampoline_refcount
+#define code_watcher_id _PyRuntime.ceval.perf.code_watcher_id
+
+static void free_code_arenas(void);
+
+static void
+perf_trampoline_reset_state(void)
+{
+    free_code_arenas();
+    if (code_watcher_id >= 0) {
+        PyCode_ClearWatcher(code_watcher_id);
+        code_watcher_id = -1;
+    }
+    extra_code_index = -1;
+}
+
+static int
+perf_trampoline_code_watcher(PyCodeEvent event, PyCodeObject *co)
+{
+    if (event != PY_CODE_EVENT_DESTROY) {
+        return 0;
+    }
+    if (extra_code_index == -1) {
+        return 0;
+    }
+    py_trampoline f = NULL;
+    int ret = _PyCode_GetExtra((PyObject *)co, extra_code_index, (void **)&f);
+    if (ret != 0 || f == NULL) {
+        return 0;
+    }
+    trampoline_refcount--;
+    if (trampoline_refcount == 0) {
+        perf_trampoline_reset_state();
+    }
+    return 0;
+}
 
 static void
 perf_map_write_entry(void *state, const void *code_addr,
@@ -407,6 +443,7 @@ py_trampoline_evaluator(PyThreadState *ts, _PyInterpreterFrame *frame,
                                    perf_code_arena->code_size, co);
         _PyCode_SetExtra((PyObject *)co, extra_code_index,
                          (void *)new_trampoline);
+        trampoline_refcount++;
         f = new_trampoline;
     }
     assert(f != NULL);
@@ -433,6 +470,7 @@ int PyUnstable_PerfTrampoline_CompileCode(PyCodeObject *co)
         }
         trampoline_api.write_state(trampoline_api.state, new_trampoline,
                                    perf_code_arena->code_size, co);
+        trampoline_refcount++;
         return _PyCode_SetExtra((PyObject *)co, extra_code_index,
                          (void *)new_trampoline);
     }
@@ -487,6 +525,10 @@ _PyPerfTrampoline_Init(int activate)
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
     PyThreadState *tstate = _PyThreadState_GET();
+    if (code_watcher_id == 0) {
+        // Initialize to -1 since 0 is a valid watcher ID
+        code_watcher_id = -1;
+    }
     if (!activate) {
         _PyInterpreterState_SetEvalFrameFunc(tstate->interp, prev_eval_frame);
         perf_status = PERF_STATUS_NO_INIT;
@@ -504,6 +546,13 @@ _PyPerfTrampoline_Init(int activate)
         if (new_code_arena() < 0) {
             return -1;
         }
+        code_watcher_id = PyCode_AddWatcher(perf_trampoline_code_watcher);
+        if (code_watcher_id < 0) {
+            PyErr_FormatUnraisable("Failed to register code watcher for perf trampoline");
+            free_code_arenas();
+            return -1;
+        }
+        trampoline_refcount = 1;  // Base refcount held by the system
         perf_status = PERF_STATUS_OK;
     }
 #endif
@@ -525,17 +574,19 @@ _PyPerfTrampoline_Fini(void)
         trampoline_api.free_state(trampoline_api.state);
         perf_trampoline_type = PERF_TRAMPOLINE_UNSET;
     }
-    extra_code_index = -1;
+
+    // Prevent new trampolines from being created
     perf_status = PERF_STATUS_NO_INIT;
-#endif
-    return 0;
-}
 
-void _PyPerfTrampoline_FreeArenas(void) {
-#ifdef PY_HAVE_PERF_TRAMPOLINE
-    free_code_arenas();
+    // Decrement base refcount. If refcount reaches 0, all code objects are already
+    // dead so clean up now. Otherwise, watcher remains active to clean up when last
+    // code object dies; extra_code_index stays valid so watcher can identify them.
+    trampoline_refcount--;
+    if (trampoline_refcount == 0) {
+        perf_trampoline_reset_state();
+    }
 #endif
-    return;
+    return 0;
 }
 
 int

Python/pylifecycle.c

@@ -1944,7 +1944,6 @@ finalize_interp_clear(PyThreadState *tstate)
         _PyArg_Fini();
         _Py_ClearFileSystemEncoding();
         _PyPerfTrampoline_Fini();
-        _PyPerfTrampoline_FreeArenas();
     }
 
     finalize_interp_types(tstate->interp);